Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Quantas voltas foram completadas na grade 18?
 ###Tabelas: CREATE TABLE title_holders_from_1992_until_1999 (title_holders_from_1992_until_1999_id AUTO_INCREMENT PRIMARY KEY, Year INT, Binibining Pilipinas-Universe VARCHAR, Binibining Pilipinas-World VARCHAR, Binibining Pilipinas International VARCHAR, Miss Maja Pilipinas VARCHAR, First runner-up VARCHAR, Second runner-up VARCHAR) CREATE TABLE nationwide_series (nationwide_series_id AUTO_INCREMENT PRIMARY KEY, Year INT, Starts INT, Wins INT, Top 5 INT, Top 10 INT, Poles INT, Avg. Start VARCHAR, Avg. Finish VARCHAR, Winnings VARCHAR, Position VARCHAR, Team(s) VARCHAR) CREATE TABLE bowling_averages_top_10 (bowling_averages_top_10_id AUTO_INCREMENT PRIMARY KEY, Player VARCHAR, Team VARCHAR, Matches INT, Overs VARCHAR, Wickets INT, Average VARCHAR, Economy VARCHAR, BBI VARCHAR, 4wi INT, 5wi INT) CREATE TABLE season_20_1974_75 (season_20_1974_75_id AUTO_INCREMENT PRIMARY KEY, Series # INT, Season # INT, Title VARCHAR, Director VARCHAR, Writer(s) VARCHAR, Airdate VARCHAR) CREATE TABLE classification (classification_id AUTO_INCREMENT PRIMARY KEY, Driver VARCHAR, Constructor VARCHAR, Laps INT, Time/Retired VARCHAR, Grid INT)  
SELECT SUM Laps FROM classification WHERE Grid = 18